I would be interested to hear your opinion (whether is a simple idea, known theory, or something completely surreal) about where the Universe stops / ends. We do know that galaxies are moving away from each other, so the universe expand, "grew up" in a way. I am talking about the place Where the universe has not yet extended, the universe is the content, I talk about the container, and in particular of this moving frontier.

One could argue that, by definition the universe contains everything that exists, including space-time (and this is an essential precision), it can have no "edge" as we intuitively Understand this concept. Indeed, the existence of an edge implies that beyond this edge, we would not be in the Universe, which would be absurd. But if the universe has no edge in the intuitive sense of the term, then its expansion is not intuitive either: if it were, what would the universe be expanding in ?

Here we see the apparent paradoxes caused by using "intuitive" notions ... which are only the reflection of a local perception of reality. To give a precise definition in these intuitive notions allows to remove these paradoxes. We see that this problem is beyond our simplistic reasonings, which base themselves on hypotheses

Well, from what I understand of this field of cosmology, there are essentially two options: either the universe is a gigantic, but finite, hypersphere, or the universe is truly infinite.

You could see the hypersphere as similar to our own Earth. It has a finite size, albeit very large compared to the creatures that live on it. Yet, despite Earth's finite size, you can travel in any direction infinitely and yet you'll never reach an edge: after 40000 km you get back to where you started. The universe as a hypersphere would essentially be the same principle, except the sphere would be four-dimensional, not three dimensional. This would mean you can go in any of the three dimensions infinitely, without ever encountering an edge, but after hundreds of billions of lightyears you'd get back to where you started.

In the other case, the universe is simply infinite, and there is no edge either: you can just keep on travelling in any direction for googolplexes of lightyears.

In either case, the universe would not really be expanding 'in' something. The galaxies move farther and farther away from each other because of the space between them expanding. But since neither of these ideas for the universe has an edge, there are no galaxies on the edge filling something that wasn't universe before either. In the case of the infinite universe, all galaxies simply get farther and farther away from each other. In the case of the hypersphere universe, it would be like a balloon with little pictures of galaxies sticked onto it that is inflated.

The best evidence we have suggests that the universe is open and "flat", not curved in 4 dimensions at all, although it could be. There are a few other odd shapes that have been proposed for the universe that cannot be ruled out. It should also be noted that the "flat" universe would also be infinite.

Why would the universe not be infinite? Empty space seems to be infinite, there is only a finite number of galaxies inside it. They have fired powerful lasers to try and find some sort of curve of a hyper-sphere but they found no curve at all. At least from what I think this seems the most logical, there are no other stars/galaxies if you go in one direction for trillions of years at high speeds, but you can still travel there. It's hard to think about.

Well, I'm aware of this two options (nice vulgarisation apenpaap,) and I have nothing against an infinite universe, my logic sense tells me that it is surely true and I have faith in science. I understand that it can not logically be any edge since there is no center. However, it is very hard for me to imagine a universe where I can travel in any direction and never reach any edge. I think that the shape of the Universe is something that still goes far beyond us (i'm not a religious man ), and that we are more or less at the same point than the men who wondered if the earth was either flat or round. i dont say that we are completely ignorant huh! Mainly just that, we know very few about it. (And even if those theories are correct, there will be still a lot of questions without answers.)

Do you guys have a more personal opinion or you all just agree with the known theories?

Quote (DoctorOfSpace)

I rather like this idea about traveling far away as it makes the most sense.

I've always had difficulties with parallels universes with copies of us. that's seem to me like a very human idea

Do you guys have a more personal opinion or you all just agree with the known theories?

I no longer generate any personal opinions on matters such as this. I have no reason to think that any particular idea is true, so I subscribe to none of them. The only thing that I will do is reject those ideas that are contradicted by confirmed observation, for then they are not true. I also feel that formulating an opinion in the absence of evidence and the absence of understanding (since this topic is somewhat beyond what I can fully grasp) might impair my ability to recognize and accept the truth should it one day be knowable.

Quote (Solaris)

I've always had difficulties with parallels universes with copies of us. that's seem to me like a very human idea

I think the problem here arises from the fact that we cannot thoroughly perceive the notion of infinity, of time and of 4 or more dimensions. Me at least <_<. All these concepts are not easy to grasp because they cannot be directly observed or measured. There is an inherent and innate limit in our brains (mine at least ) that makes it very difficult to perceive these theories. For example: Regarding parallel universes that are like membranes parallel to each other: In what medium are these universes suspended in??? Is this a hyper-universe??What is it made of?? And if it is, is it contained in another hyper-hyper universe??? And it goes on and on .... Consider our galaxy for example: It has a diameter of 100000-200000 Lys, It contains 200-400 billions of stars. For me our galaxy is a universe of its own . Even if we manage to travel at close to speed of light we will need billions of years to explore it. And this can be perfevtly illustrated in SE!! There was an argument in another game forum regarding the game ''ïnfinity". Many people(and myself) were arguing why ''infinity'' will ''only'' feature our galaxy. Well I believe the answer is obvious now . I am not saying that space engineer should make SE with only the milky way . On the contrary: SE is the ONLY simulation software that makes it possible to go where no other sim has gone before: To the edge of the known universe!!

how many genius were considered as loonies before that their theories become standards?

I always allow room for the improbable and the strange. The only things I will discount are things that have been positively proven false. If something seems unlikely, but has not been disproven, then I will not completely dismiss it.

Quote (neutronium76)

For me our galaxy is a universe of its own

When it was first discovered what the galaxies were in the 1920s, they were actually referred to as "island universes"

Things in nature at nearly every scale are incomprehensibly vast. Atoms are amazingly complex. Microscopic life forms are so much more complex than that. Large beings like humans are made of trillions of those stupendously complex cells. Human society is made of billions of individuals interacting in very complex ways on every scale. Humans are a very small part of the Earth's biosphere. The biosphere is insignificant compared to the whole of our planet. Earth is barely more than a speck of dust in an average star system. That star system is one of over 100 billion in a vast ocean of stars, gas, and dust that we called the Milky Way galaxy. And in just the part of the universe that we can observe, our galaxy is as insignificant as our solar system is to the galaxy.

There are "universes" of complexity and scope to boggle the mind at every scale

But I agree with you, I am also not comfortable thinking too deeply about the universe on a scale larger than our galaxy.

I've always had difficulties with parallels universes with copies of us. that's seem to me like a very human idea

We are talking scales of infinity and simple probability though. If our universe exists and a potential of an infinite amount of other universes exist then simply put there are an infinite other Earths in the multiverse. Thats not really a very human idea when the human idea was each of us is supposedly unique.

Your last post resume very well how insignificant we are and how complex this is , HarbingerDawn. And I like the island universes term quite much !

Quote (neutronium76)

There is an inherent and innate limit in our brains

Yes but doesn't that limit extend all over the time ? I mean, like microbes, people could not conceive of such miniature worlds, yet today it is normal, assimilated. Maybe one day, notion of infinity, time and 4 dimensions will be taught in schools like history or whatever..And assimilated too. Yes we are small , but not only, our science has understood many things very well, I say that maybe we could understood very much more things while as our specie evolve. we have no way of knowing what will become our specie, only speculate (and here I say we for sure aren't in the good direction..)

Quote (neutronium76)

For example: Regarding parallel universes that are like membranes parallel to each other: In what medium are these universes suspended in???

I wanna know ! That is the kind of place I was talking about.

Quote (DoctorOfSpace)

We are talking scales of infinity and simple probability though.

All is here, I do not say that such thing cant exist, there is so much differents probability here, and they found a way to pick the one where everyone is duplicate. whynot. anyway, still a attractive theorie !

Quote (DoctorOfSpace)

Thats not really a very human idea when the human idea was each of us is supposedly unique.

I've always had difficulties with parallels universes with copies of us. that's seem to me like a very human idea

I'd agree with this.

Quote (DoctorOfSpace)

If our universe exists and a potential of an infinite amount of other universes exist then simply put there are an infinite other Earths in the multiverse.

How does that mean there are infinite amounts of other universes out there? If there are infinite amounts of universes then wouldn't that mean that there are so many that at least some of them would have a civilization that somehow broke out of their universe to visit another one, or some of them would visit our Earth? Infinity is an extremely crazy thing to comprehend and it allows paradox' like.

If there are infinite other universes then there are an infinite amount of times where that universe blew up and destroyed ours.

And if it's "impossible" to leave a universe or enter another one or send any form of information from other universes then how can we detect other universes? If there's no way we can detect, see, record, visit any signs of unlimited universes then what is the point of thinking that's how it is?

I think it's far more likely that we're actually in a simulation for another civilization that created Space Engine 8.93, which could then be in a simulation, and so on and so on, as according to probability we're very unlikely to be in the "Original" universe. I don't really believe this though, until we observe evidence of it, but it is possible.